1. Field of the Invention
The invention herein relates to a system and method for catalytically reducing the content of nitrogen oxide in a flue or stack gas, resulting from the combustion of fuel in a furnace
2. Description of the Related Art
The combustion of fuels in various industrial processes often generates undesirable oxides of nitrogen (NOx), usually in the form of nitric oxide (NO) and nitrogen dioxide (NO2). High combustion temperatures tend to produce more NOx. Because NOxis harmful to the environment, efforts have been made to reduce the emission of NOx in gases produced by industrial processes involving the combustion of fuel, particularly gases resulting from the operation of power plants, thermal cracking furnaces, incinerators, internal combustion engines, metallurgical plants, fertilizer plants and chemical plants.
Methods for selectively reducing the NOx, content of a flue gas are known. Generally, such methods involve the reaction of NOx with a reducing agent, optionally in the presence of a catalyst. The selective non-catalytic reduction (xe2x80x9cSNCRxe2x80x9d) of NOx with a reducing agent such as ammonia or urea requires a relatively high temperature, e.g., in the range of from about 1600xc2x0 F. to about 2100xc2x0 F.
Alternatively, the reduction of NOx with ammonia can be performed catalytically at a much lower temperature, e.g. from about 500xc2x0 F. to about 950xc2x0 F., in a process known as selective catalytic reduction (xe2x80x9cSCRxe2x80x9d).
One problem associated with the treatment of flue gas using conventional SCR methods and apparatus is that the weight and bulk of the equipment necessary to achieve satisfactory removal of NOx requires that it be located at ground level. Many industrial plants need to be retrofitted with NOx removal (xe2x80x9cdeNOxxe2x80x9d) equipment in order to meet the requirements of more stringent government regulations. However, because of the physical bulk of the deNOx system, the flue gas must be diverted to ground level for treatment and then sent back into a stack for subsequent exhaust to the atmosphere. To avoid the large cost of such a system it would be highly advantageous to provide a relatively lightweight deNOx unit which can be incorporated directly into the stack.
In accordance with the present invention, a system is provided for the selective catalytic reduction of nitrogen oxide in a furnace gas stream and comprises:
a) a first heat recovery section positioned in a stack above the furnace for reducing the temperature of the furnace gas stream exiting the furnace to not less than about 400xc2x0 F.;
b) a reactor for the selective catalytic reduction of nitrogen oxide in the gas stream positioned in the stack downstream of the first heat recovery section, said reactor including an injector for introducing a reducing agent into the gas stream and containing at least one nitrogen oxide conversion catalyst for the selective catalytic reduction of nitrogen oxide in the gas stream upon contact therewith to provide a treated gas stream of reduced nitrogen oxide concentration;
c) a treated gas stream outlet through which the treated gas stream is discharged; and,
d) a second heat recovery section positioned downstream of the reactor for reducing the temperature of treated gas.